1. Field of the Invention
The invention relates to a pressure wave sensing device of the piezoelectric type and a method for manufacturing the same.
The device of the invention finds applications particularly in the field of seismic prospection where reception devices are used formed of a number of seismic receivers, often a high number, disposed at intervals from each other and connected to a central recording laboratory. With acoustic waves emitted into the ground, the signals which have been reflected or refracted from the different interfaces of the subsoil are picked up by this receiver array. By recording and processing the signals picked up, a set of traces is obtained representative of the configuration of the subsoil, as is well known.
Each receiver includes an elementary sensor or several sensors interconnected in series and/or in parallel for increasing their voltage and load sensitivity respectively. For marine applications, the receivers are disposed along a sealed sheath or seismic streamer towed immersed by a ship.
2. Description of the Prior Art
The sensors currently used are of the piezoelectric type. They include disks of relatively small dimensions made from a piezoelectric material and each is associated with a pair of electrodes. These disks are contained in cases having one or more faces transparent to the acoustic waves. Such sensors are described for example in the U.S. Pat. No. 3,489,995.
From French Pat. No. 2.145.099 or U.S. Pat. No. 3,798,474 a sensor device is known formed of at least one sensitive element in the form of a strip. This strip is formed of a flexible ribbon made from a piezoelectric material and two electrodes on each side of the ribbon and, more generally, several sensitive elements connected in series so as to increase the electric voltage sensitivity of the assembly. In one arrangement, a seismic receiver is formed by winding a first sensitive strip associated with its two electrodes on a cylindrical insulating core. A flexible insulating strip is superimposed on the first sensitive strip by winding. Over that is again wound a second sensitive strip in a spiral in the opposite direction to the first one. Rings in contact respectively with the electrodes of the two sensitive strips are connected by conducting wires so as to form a series interconnection.
In its practical embodiment, such an arrangement has drawbacks. The electrical connection of the electrodes of the two sensitive strips is generally provided at the ends of the support through conducting rings. Since the resistance of the conducting films forming the electrodes is not negligible, in a parallel combination of the two sensitive strips for example, the length of the electrodes may contribute to limiting the pass band of the sensor
Furthermore, it does not lend itself well to industrial production to the extent that the collecting rings at each end of each receiver section are positioned in several different manufacturing steps so as to be in contact with the electrodes which correspond therewith before winding other layers.
The sensor device and the manufacturing method of the present invention avoid the above mentioned drawbacks.